This invention relates to a process for separating the chlorination products from the reaction gases obtained by the chlorination of methane and methyl chloride.
In the manufacture of methyl chloride, methylene chloride, chloroform, and carbon tetrachloride, chlorine is reacted in the gaseous phase with excess methane and optionally also methyl chloride. Conventionally, the hydrogen chloride in the product gas stream formed during the chlorination is first separated by absorption with 20% strength hydrochloric acid and water and the remaining gas is then dried. The chlorinated hydrocarbons, in most cases, after compression of the gas are then condensed out of the product gas stream by cooling with cooling water and cooling brine, or they are washed out with low-temperature carbon tetrachloride and/or chloroform. Hydrocarbons are also utilized as absorption agents. Depending on the condensation temperature and amount of washing agent, a more or less large amount of methyl chloride remains in the gas which, after withdrawing a small amount of waste gas to prevent build-up of inert components, is recycled to the reactor as cycle gas. If a high yield of methylene chloride is desired, which is generally the case, no methylene chloride is left in the cycle gas, since it would otherwise be further chlorinated to chloroform and carbon tetrachloride. Since very often a higher proportion of methylene chloride is required than is obtained by the chlorination of methane, methyl chloride which had already been separated from the product gases by condensation or washing is recycled to the reactor after the distillative elimination of higher chlorinated components. Sometimes not only the entire methyl chloride produced by the chlorination but also extraneous methyl chloride obtained according to other processes, e.g., by reaction of hydrogen chloride with methanol, is fed to the chlorination reactor.
In the method of Farbwerke Hoechst (Ullmann, vol. 5, 1954, p. 404; Winnacker-Kuechler, "Chemische Technologie", vol. 4, 1972, p. 46), the reaction gas is first freed of the hydrogen chloride and then subjected to a low-temperature condensation. The gaseous stream leaving the condenser is recycled as cycle gas to the reactor after first branching off an exhaust gas stream to eliminate the inert components thereof. The condensed-out chlorinated hydrocarbons are then subjected to a distillation. Excess methyl chloride from the distillation plant, which is to be further chlorinated, is introduced into the cycle gas upstream of the reactor.
In the process according to Chemische Werke Huels AG. (Ullmann, vol. 5, 1954, p. 406; Winnacker-Kuechler, "Chemische Technologie," vol. 4, 1972, p. 46), the reaction gas, after separation of the hydrogen chloride, is compressed to about 8 bars, and is then condensed at about -13.degree. C. The cycle gas is fed to the chlorination reactor in the same manner as in the method by Farbwerke Hoechst.
In the process in accordance with DOW Chemical Corp. (Ullmann, vol. 5, 1954, p. 406; Winnacker-Kuechler, vol. 4, 1972, p. 46), the chlorinated hydrocarbons are washed out of the reaction gas, prior to separation of the hydrogen chloride, by absorption with low-temperature-cooled chloroform and carbon tetrachloride. The low-chlorinated hydrocarbons are removed by distillation, then fed in the gaseous phase to a water washing stage in order to separate the hydrogen chloride, and then compressed and fractionated in a pressure distillation stage.
In another modification of working up the reaction gas described in German Published Application (DAS) No. 1,568,575, after cooling and compression, the reaction gas is scrubbed by washing with precooled, monochlorinated reaction products, in the case of a mixture of methane and methyl chloride, in one or two stages to separate the hydrogen chloride and the chlorinated hydrocarbons.
All of the above-mentioned processes have the feature in common that the condensation and/or absorption is conducted at low temperatures. When none or only a small amount of methyl chloride is desired as a product, the excess portion of the thus-condensed or absorbed methyl chloride must therefore be reevaporated and fed to the cycle gas upstream of the reactor. The advantage attained in the process according to DAS No. 1,568,575, viz., the direct production of dry hydrogen chloride, is confronted by considerable disadvantages, such as operating at low temperatures, higher pressures, and the compression of a gas containing hydrogen chloride, as well as, when methylene chloride is a desired reaction product, the energy-consuming evaporation of the thus-condensed methyl chloride.
Since methyl chloride can also be obtained in another, very simple process by reaction of methanol with hydrogen chloride (hydrochlorination), methyl chloride from this hydrochlorination in admixture with methane is used with increasing frequency as the starting material for producing methylene chloride and chloroform.
The present invention is directed to the problem of eliminating, insofar as possible, the disadvantage of the expensive condensation of methyl chloride and its subsequent evaporation, and especially avoiding the use of low temperatures without losing flexibility with regard to the product spectrum.